1. Field of the Invention
This invention relates in general to subsea well drilling, and in particular to a means for preventing an inner wellhead housing secured to the lower end of a riser suspended from a drilling vessel from rotating within a conductor or an outer wellhead housing.
2. Description of the Related Art
Many subsea wells are drilled by first drilling a large diameter hole, then installing a string of conductor pipe, which has an outer wellhead housing secured to the upper end. Then, the operator drills the well to a greater depth and installs a first string of casing. An inner wellhead housing secures to the upper end of the string of casing and lands within the outer wellhead housing. The operator will then drill the well to a further depth. A string of riser will extend from the inner wellhead housing to the drilling vessel.
A floating drilling vessel can cause rotational forces on the riser. Normally, the rotation is resisted by frictional engagement of the landing shoulders of the inner wellhead housing and the outer wellhead housing. If the rotational force is high enough to cause the inner wellhead housing to begin to rotate within the outer wellhead housing, one of the casing joints below the inner wellhead housing could start to unscrew, causing a serious problem.
An anti-rotation device is provided to prevent an inner wellhead housing from rotating within an outer wellhead housing. The anti-rotation device includes providing a plurality of anti-rotational keys between the inner and outer wellhead housing. In a first embodiment the keys face inwards to the inner wellhead housing and are circumferentially spaced apart around the outer wellhead housing located between the two tapered shoulders. The anti-rotational device additionally includes providing a plurality of anti-rotational mating slots located on the exterior of the inner wellhead housing. The plurality of keys are spring loaded and extend radially outward from the outer wellhead housing in an extended position. As the inner wellhead housing lands in the outer housing, the inner wellhead housing pushes the keys of the outer wellhead housing into a retracted position. The inner wellhead housing is then rotated within the outer wellhead housing until the spring loaded keys align with the slots and extend into the slots of the inner wellhead housing. Any rotational force on the inner wellhead housing will be resisted by the anti-rotational mechanism. The control of rotational resistance may be controlled be varying the number keys and slots.
In the second embodiment, the keys face outwards to the outer wellhead housing and are circumferentially spaced apart around the inner wellhead housing located between the two tapered shoulders. The anti-rotational device additionally includes providing a plurality of anti-rotational mating slots located on the interior of the outer wellhead housing. The plurality of keys are spring loaded and extend radially outward from the inner wellhead housing in an extended position. As the inner wellhead housing lands in the outer housing, the outer wellhead housing pushes the keys of the inner wellhead housing into a retracted position. The inner wellhead housing is then rotated within the outer wellhead housing until the spring loaded keys align with the slots and extend into the slots of the outer wellhead housing. Any rotational force on the inner wellhead housing will be resisted by the anti-rotational mechanism. The control of rotational resistance may be controlled be varying the number keys and slots.